An optical attenuator is generally utilized to attenuate the intensity of a light beam in a predetermined ratio. For example, an optical attenuator may be used in a measurement or other application to attenuate the intensity of an output light beam from a light source such as a high power semiconductor laser diode. A conventional optical attenuator typically includes one or more ND (Neutral Density) filters. It is possible to provide such an optical attenuator with a desired transmittance (the reciprocal of the attenuation ratio) by selectively combining a plurality of ND filters each having a predetermined transmittance.
The attenuation function of an ND filter in such an optical attenuator is obtained by reflecting an incident light beam from a light source with a predetermined reflectance. However, light returned from the optical attenuator to the light source causes unstable operation of the laser diode in the light source due to the optical feedback effect.
It is conventional to slightly incline such ND filters with respect to the incident light beam to prevent the reflected light from returning to the light source. In order to ensure that reflected light is not returned to the light source, the filters must be spaced from the light source by a distance that depends on the width of the light beam. Since a typical semiconductor laser diode emits a divergent light beam, the output laser beam inevitably has a rather large diameter even if the beam is collimated by a collimator lens. Therefore, it is difficult to make an optical attenuator that is small in size.
If the angle at which the filters are inclined to the optical axis is large, it may be possible to prevent reflected light from returning to the light source without placing the filters far away from the light source. If the angle of the filters is too large, however, the polarization effect of the filter is changed. Thus, the transmittance of the filters is changed in response to the polarized direction of the incident light beam as described hereinafter. According to the theory of optics, when linearly polarized light is incident on a medium such as glass at a particular angle of incidence Th, the transmittance T of ,the medium is: EQU T=TP* cos.sup.2 A+TS* sin.sup.2 A (1)
where A is an azimuthal angle which is defined as the angle between the electric field vector of the incident light beam and the plane of incidence. The plane of incidence is defined as the plane including both the axis of the incident light beam and the normal to the surface of the medium at the point of incidence. TP is the transmittance of the component parallel to the plane of incidence and TS is the transmittance of the component perpendicular to the plane of incidence. The symbol * represents the multiplication operation.
Since the values of TP and TS are dependent on the angle of incidence Th, the values of TP and TS differ increasingly from each other as the angle of incidence Th increases. When the light source revolves around the optical axis, and therefore the azimuthal angle changes, the transmittance T is changed so that the intensity of the transmitted light is changed. Thus, since it is undesirable to considerably incline the filters with respect to the incident beam, it is conventional to provide a unidirectional apparatus such as an optical isolator between the light source and the filters to prevent reflected light from returning to the light source. This is a significant problem causing complexity and inaccuracy of a conventional optical attenuator.